1. Field of the Invention
This invention relates generally to a method and topology for a resonant inverter. Specifically, the resonant inverter is capable of being incorporated in a charger that wirelessly charges the energy storage device of an electric vehicle or an electronic device.
2. Background Discussion
There are growing interests in wireless charging batteries of electronic devices, such as phones and electric vehicles. For the proliferation of electric vehicles, especially in the urban environment, it is desirable that charging instruments are accessible as widely as possible. While it may be a routine for an electric vehicle to go to a dedicated charging station to obtain energy, it would be more convenient if an electric vehicle may be charged while parking or even waiting for traffic signals. Technologies related to a wireless power transfer have been under development for many years to solve the charging demand of electronic devices. Two wireless power transfer technologies for charging electric vehicles have been under research: one is inductive charging, and the other is charging via magnetic resonance.
Inductive charging has already been known and well described in many patents and publications. The inductive charging technology has, however, a significant disadvantage. It works well only at a very short distance between a transmitter and a receiver, which requires precise device positioning and complicated mechanical solutions, which is probably a main reason why there is not much interest in the inductive charging technology in the wireless-charging market of electric vehicles.
The magnetic resonance technology utilizes inductors connected with capacitors in a resonant circuit which allows for compensating a large leakage inductance of the coupled inductors. It allows for operating with significant air gaps between transmitting and receiving windings. In other words, the transmitter and receiver form a coreless connection. Magnetic resonance utilizes inductors tuned for the same resonant frequency to facilitate wireless power transfer. From an electrical point of view, the technology is based on a well-known technique for compensation of a leakage inductance of a transformer. The transmitted power and efficiency change significantly with operating frequency and circuit parameters due to the use of resonant circuits. That is why magnetic resonance technology requires dedicated power electronics solutions like specialized resonant inverters.
In general, resonant inverters convert a direct current to an alternating current by using a resonant circuit. Conventional control methods for resonant converters use either frequency or phase to adjust the output of the circuit. Optimization of the design of the inverter for this particular technology and application can help to achieve outstanding performance.
Frequency control is a popular method and can be implemented for a wide variety of resonant converter topologies. Many of the techniques are described and analyzed in Reference [1]. Frequency control, regardless of its simplicity has some disadvantages. It requires a wide range of operating frequency to regulate output and a very high switching frequency at light loads. Compared with frequency control, phase control can avoid those problems associated with frequency control. Phase control typically operates at a constant frequency and can obtain a zero-voltage-switching (ZVS) condition over an entire regulation range. Such inverters with different resonant circuits are described and analyzed in References [2]-[4]. One can see that these inverters consist of two half-bridges connected in parallel. Generalization of this concept leads to paralleling multiple phases, which is presented in Reference [5].
Other approaches to improve the performance of multiphase resonant converters include connecting them in parallel downstream of the rectifier. This approach gives several new control and ripple reduction possibilities. Such approach is shown and analyzed in References [6]-[12].